Please document the validation rules precisely

[CrispinF]

@janputzan please could we have specific documentation on the validation tests being undertaken?

For example on
https://github.com/HistoricEngland/H.API/blob/master/ValidationRules.md#digitalobjectidentifier
could you specify the exact test being undertaken to check "that the field contains a valid DOI".
The schema documentation currently says "Acceptable Values: Must have text segment "doi" in URL" however we're aware from the recent testing that the actual validation test is more specific than that.

And likewise for every other element/rule? e.g. in testing we've seen a validation test being done on endDate that is not described in the documentation.

Thanks!